Field of the Invention
The present invention relates to a piezoelectric film and piezoelectric ceramics.
Background Art
FIG. 8(A) is a graph showing a DE hysteresis curve and a piezoelectric butterfly curve of a Pb(Zr, Ti)O3 film (hereinafter, referred to as “PZT film”) having a thickness of 2 μm, and FIG. 8(B) is a graph showing the DE hysteresis curve and the piezoelectric butterfly curve of the PZT film having a thickness of 4 μm.
The PZT films having thicknesses of 2 μm and 4 μm are produced in the following way.
A Pt film oriented in the (100) plane on a wafer of 4 inches, and on the Pt film a PZT film is formed by a sputtering method (for example, refer to Patent Literature 1). One example of the sputtering conditions is as follows. By controlling a film forming time of the PZT film, Sample 1 in which the PZT film having a thickness of 2 μm and Sample 2 in which the PZT film having a thickness of 4 μm are produced. Then, the evaluation of hysteresis and the evaluation of piezoelectric butterfly of each PZT film of Sample 1 and Sample 2 are performed.
[Sputtering Conditions]
Machine: RF Magnetron sputtering apparatus
Power: 1500 W
Gas: Ar/O2 
Pressure: 0.14 Pa
Temperature: 600° C.
Film forming rate: 0.63 nm/sec
FIG. 9 is a graph which is the same as of FIG. 8(A) and which shows that the voltage at which the piezoelectric butterfly curve takes the minimum value is almost the same as the coercive voltage Vc of the DE hysteresis curve. It can be read from FIG. 8(B) that the voltage at which the piezoelectric butterfly curve also takes the minimum value is almost the same as the coercive voltage Vc of the DE hysteresis curve. In this way, according to the conventional piezoelectric film, the piezoelectric butterfly (mechanical vibration) property and the ferroelectric hysteresis property (movement of electrons) are almost the same.
FIG. 10(A) is a graph showing the piezoelectric hysteresis curve obtained by applying a voltage of ±5 V, 700 Hz to a PZT film having a thickness of 2 μm (Sample 1) and thus causing the film to perform bipolar driving, and FIG. 10(B) is a graph showing the piezoelectric hysteresis curve obtained by applying a voltage of ±10 V, 700 Hz to a PZT film having a thickness of 2 μm (Sample 1) and thus causing the film to perform bipolar driving.
When applying a voltage of ±5 V which is not larger than the coercive voltage Vc of the DE hysteresis curve to the PZT film having a thickness of 2 μm shown in FIG. 8(A) and thus causing the film to perform bipolar driving, the piezoelectric hysteresis property shows the linear behavior as shown in FIG. 10(A). However, when applying a voltage of ±10 V which is larger than the coercive voltage Vc of the DE hysteresis curve to the PZT film having a thickness of 2 μm shown in FIG. 8(A) and thus causing the film to perform bipolar driving, an electrostrictive factor is included in the piezoelectric hysteresis property as shown in FIG. 10(B).
FIG. 11(A) is a graph showing the piezoelectric hysteresis curve obtained by applying a voltage of ±5 V, 700 Hz to a PZT film having a thickness of 4 μm (Sample 2) and thus causing the film to perform bipolar driving, and FIG. 11(B) is a graph showing the piezoelectric hysteresis curve obtained by applying a voltage of ±10 V, 700 Hz to a PZT film having a thickness of 4 μm (Sample 2) and thus causing the film to perform bipolar driving.
When applying a voltage of ±10 V which is not larger than the coercive voltage Vc of the DE hysteresis curve to the PZT film having a thickness of 4 μm shown in FIG. 8(B) and thus causing the film to perform bipolar driving, the piezoelectric hysteresis property shows the linear behavior as shown in FIG. 11(B). In other words, even at a high voltage of ±10 V at which the electrostrictive factor is included in the piezoelectric hysteresis property in the case of a thickness of 2 μm by making a thickness of the PZT film as large as 4 μm, it becomes possible to cause the film to perform bipolar driving in which the piezoelectric hysteresis property shows linear behavior. Namely, in order to cause the conventional piezoelectric film to perform a piezoelectric operation at a high voltage, it is necessary to make the film thickness large for gaining the coercive voltage Vc.